1. Field of the Invention
The present invention relates to imaging systems, more particularly, to personnel body scan systems for the detection of contraband, such as explosives, guns, and the like.
2. Description of the Related Art
Backscatter x-ray systems have been in use for at least three decades. The low x-ray energy version of these systems typically use a scanning pencil beam of x-rays. A cone beam of x-rays emitted by an x-ray source impinges on a fixed horizontal slot to form a horizontal fan beam. The fan beam impinges on radial slits in the vertically rotating disk to produce the pencil beam that scans horizontally across the person. The x-rays in the pencil beam penetrate the clothing of the person with very little absorption and are absorbed or scattered by the much thicker body. The x-rays scattered from each location along the scan line are detected by a large area backscatter detector and form one horizontal line of the image. The entire assembly, including the x-ray source, the pencil beam-forming mechanism and backscatter detector, is then translated vertically to form a two-dimensional image of the entire body. Contraband is seen as an area of contrast on top of the body image.
The peak x-ray voltage is chosen to be large enough to penetrate the person's clothing and small enough so as to not penetrate the person's body excessively. Thus the images appear to strip the person of his clothing. Low atomic number (low Z) materials, e.g., explosives, scatter back more X-rays, so they are seen as white. High atomic number (high Z) materials, e.g., guns, scatter back very few x-rays, so they are seen as black.
The United States government has established a maximum incident x-ray flux that is permitted for these systems of 10 microRoentgens (μRoentgens) per inspection measured as defined in the regulation. This restriction on the incident flux limits both the contrast and spatial resolution of these systems. Thus the image quality of the current state-of-the-art systems is poor. Thin and/or small area threats are difficult to identify or even to detect.
Backscatter imaging systems have the disadvantage of limited penetration of the subject because of the use of a pencil beam containing only a small fraction of the x-ray source emission, the relatively small probability of the scatter event and the requirement that the lower energy scattered x-ray photons must penetrate back through the subject. In the case of personnel backscatter systems, this latter requirement is not important because the contraband is located on the surface of the person's body.
Despite these disadvantages, backscatter systems for packages have a significant market because the images have a three-dimensional quality that is very pleasant to the operator.
The use of dual energy in transmission x-ray inspection systems is well known. These systems image the subject almost simultaneously with two x-ray fan beams of differing peak x-ray energies. They typically use one beam of lower peak energy of about 70 KeV and another beam of higher peak energy of about 150 KeV. The lower peak energy must be high enough to penetrate the package and low enough to differentiate between explosives or drugs and innocent plastics of nearly the same atomic number, Z. The typical requirement is to differentiate plastics and other common materials that do not contain any elements with Z>8, the atomic number for oxygen, and drugs or explosives that have the same low Z elements in different proportions and typically a large fraction of nitrogen, Z=7, a common component of drugs and explosives. These low Z elements differ very little in their attenuation of x-rays except for x-rays of very low energies, hence the need for an x-ray beam of low peak energy. The dual-energy systems using transmission x-rays do not work very well unless the thickness of the package being inspected is less than 5 cm of material of density 1 g/cm2, since the lower energy beam is rapidly absorbed.